The invention generally relates to gas concentrators, and more particularly relates to medical oxygen concentrators used by patients in the home care setting where predictive maintenance resulting in little or no unplanned downtime and minimized service cost is essential.
The application of oxygen concentrators for therapeutic use is known, and many variants of such devices exist. A particularly useful class of oxygen concentrators is designed to be portable, allowing users to move about and to travel for extended periods of time without the need to carry a supply of stored oxygen or to have any maintenance performed on their equipment by a service technician. These portable oxygen concentrators typically weigh 2 to 20 lbs and produce from 0.3 to 5.0 LPM of oxygen. Most of these portable concentrators are based on Pressure Swing Adsorption (PSA), Vacuum Pressure Swing Adsorption (VPSA), or Vacuum Swing Adsorption (VSA) designs which use one or more pumps to move air through selective adsorption beds at various adsorption and desoroption pressures. In a typical oxygen concentrator, the beds utilize a zeolite adsorbent to selectively adsorb nitrogen, resulting in pressurized, oxygen-rich product gas.
The main elements in a typical portable therapeutic oxygen concentrator are shown in FIG. 1. Air is draw in, and typically filtered, at air inlet 1 before being pressurized by compressor 2 to a pressure of 1.2 to 2.5 atmospheres. The pressurized air is directed by a valve arrangement through adsorbent beds 3. An exemplary adsorbent bed implementation, used in a concentrator design developed by the inventors, is two columns filled with a lithium exchanged zeolite adsorbent in the ratio of about 1 gram of adsorbent per 1-20 ml of oxygen produced. The pressurized air is directed through these adsorber vessels or columns in a series of steps which constitute a gas separation cycle, often a PSA cycle or some variation including vacuum instead of, or in conjunction with, compression yielding overall compression ratios of about 1.5:1 to 5.5:1. Although many different arrangements of adsorber vessels and gas separation cycles are possible, the result is that nitrogen is removed by the adsorbent material and the resulting oxygen rich gas is routed to a product gas storage device at 4. Some of the oxygen product gas can be routed back through the bed to flush out (purge) the adsorbed nitrogen to an exhaust 6. Generally multiple adsorbent beds, or columns in the exemplary device, are used so at least one bed may be used to make product while at least one other bed is being purged, ensuring a continuous flow of product gas. The purged gas is exhausted from the concentrator at the exhaust 6.
Such gas separation systems are known in the art, and it is appreciated that the gas flow control through the compressor and the adsorbent beds is complex and requires precise timing and control of parameters such as pressure, flow rate, and temperature to attain the desired oxygen concentration of 80% to 95% purity in the product gas stream. Accordingly, most modern concentrators also have a programmable controller 5, typically a microprocessor, to monitor and control the various operating parameters of the gas separation cycle. In particular, the controller controls the timing and operation of the various valves used to cycle the beds through feed, purge, and pressure equalization steps which make up the gas separation cycle. Also present in most portable concentrators is a conserver 7 which acts to ensure that oxygen rich gas is only delivered to a patient during inhalation. Thus, less product gas is delivered than by means of a continuous flow arrangement, thereby allowing for smaller, lighter concentrator designs. A pulse of oxygen rich air, called a bolus, is delivered in response to a detected breath via the conserver. Using a conserver in conjunction with a gas concentrator may reduce the amount of oxygen required to maintain patient oxygen saturation by a factor of about 2:1 to 9:1. A typical concentrator will also contain a user/data interface 8 including elements such as an LCD display, alarm LEDs, audible buzzers, and control buttons. In addition to the above subsystems, most portable oxygen concentrators contain a rechargeable battery and charging system to power the concentrator while away from an AC or DC power source. These battery systems are typically composed of lithium ion cells and can power the concentrator from 0.5-12 hours depending on the amount of oxygen required by the patient, device efficiency, and the capacity of the battery pack which may range from about 40 Watt-hours to 250 Watt-hours on systems with multiple battery packs.
To be practical and usable by a individual needing therapeutic oxygen, portable oxygen concentrators should be less than about 2100 cubic inches and preferably less than 600 cubic inches in total volume, less than about 20 pounds and preferably less than 8 pounds in weight, and produce less than about 45 decibels of audible noise, while retaining the capacity to produce a flow of product gas adequate to provide for a patient's oxygen needs, usually a flow rate prescribed by a medical practitioner in about the range of 1 LPM to 6 LPM. Further, a portable medical oxygen concentrator must work under varied environmental conditions such as 0° C. to 40° C. and 0%-95% relative humidity without costly or frequent service or maintenance requirements. Although fixed site PSA based concentrators have been available for many years, such fixed site units may weigh 30-50 pounds or more, be several cubic feet in volume, and produce sound levels greater than 45 dBA. Thus portable concentrators involve a significant amount of miniaturization, leading to smaller, more complex designs compared to stationary units. System size, weight, and complexity may lead to fewer mitigative options or design choices against contamination and other wear and tear effects that can lead to an unacceptably short maintenance interval.
One particular challenge of portable concentrator design is that the adsorbent beds must by necessity be small, yet capable of producing an adequate quantity of product gas. A portable oxygen concentrator might require oxygen production of greater than 3 ml of oxygen per gram of adsorbent in order to achieve an acceptable size of less than 600 cubic inches. Since the adsorbent beds are optimized for O2 production per gram of adsorbent, any significant decrease in capacity of the beds over time can result in decreased product purity as the required O2 production per gram of active adsorbent exceeds the limits of the adsorbent and PSA cycle operating parameters. One contributing factor that can lead to a decrease in bed capacity is the adsorption of impurities that do not completely desorb during normal process operation, leading to the accumulation and retention of impurities in the beds and therefore less active adsorbent than originally intended in the design. An example of such an impurity that reduces the adsorption capacity of many zeolites used in air separation is water. Some stationary concentrators utilize some means of removing water from the compressed gas before feeding the adsorbent beds, but most rely on an excess quantity of adsorbent to allow for contamination over time. Portable concentrators, by the nature of their application, are more likely to be exposed to a wide range of operating conditions including high humidity environments and/or rapid temperature changes that could result in the need for more frequent zeolite replacement. If water is present, either in the form of liquid (condensed out of the feed stream) or vapor, and enters the molecular sieve beds, the beds will irreversibly adsorb at least some of this water during each adsorption cycle. The energy of adsorption of water on lithium exchanged zeolites used in air separation is very high and not all water adsorbed during the adsorption steps in the process is desorbed during evacuation/purge of the beds under typical PSA cycle operating parameters. Therefore, complete removal of adsorbed water from zeolite beds usually entails applying some sort of energy to the beds, such as thermal, infrared, or microwave, and purging with a dry gas or applying a vacuum to the beds during the regeneration process. These regeneration processes are impractical in a portable concentrator due to high temperature or high power requirements as well as other design restraints, such as weight and size. As a result, the accumulation of adsorbed water over time results in a reduction in capacity of the beds, as fewer sites are available for nitrogen binding. Fewer binding sites in the adsorbent bed can result in a decrease in product purity over time as nitrogen passes through the sieve beds and dilutes the oxygen product gas, and shortens the service life of the concentrator. Many zeolites used in air separation, and in particular advanced adsorbents, particularly the high lithium containing low silica X type zeolite (LiLSX) used in portable concentrators, are hydrophilic in their activated state due to the interaction of the strong dipole moment of water molecules with the electric fields present in the LiLSX cages and can therefore be prone to this problem. In the effort to make more compact and efficient concentrators, PSA cycle frequencies can increase to rates approaching 10 cycles per minute and adsorbent productivity increases accordingly with advances in process and adsorbent technology to productivities exceeding 10.0 ml of oxygen per gram of adsorbent. The corresponding decrease in adsorbent inventory exacerbates the problem as the amount of gas processed per unit of adsorbent increases proportionally, (the bed size factor decreases) and the presence of impurities in the process gas can deactivate the adsorbents at a much faster rate than with conventional PSA processes, as described in U.S. Pat. Nos. 7,037,358 and 7,160,367, and U.S. application Ser. No. 13/066,716 which are incorporated by reference herein. The portable oxygen concentrator's duty cycle as well as operational and storage conditions can also impact the rate of adsorbent deactivation. When a concentrator is not running and no purge or evacuation cycling takes place, any moisture or other contaminants in a portion of a bed will diffuse into the rest of the bed, further decreasing the operational life of the bed.
It is therefore necessary to design portable oxygen concentrators such that zeolite contamination can be prevented or handled in a manner that avoids costly or frequent complex maintenance by a field technician or equipment provider. While the inventors have previously disclosed a system that achieves long sieve bed life by removing water prior to the feed gas contacting the zeolite in U.S. co-pending application Ser. No. 11/998,389, whose teachings are incorporated by reference, this approach adds size and cost to the system to achieve its resistance to zeolite contamination. It is therefore desirable to design a portable oxygen concentrator that minimizes size and weight as a function of oxygen output with commonly available LiLSX and LiX adsorbents from suppliers such as UOP, Zeochem, or Ceca. While eliminating water removal components such as membrane air dryers or pretreatment layers such as activated alumina or an NaX type zeolite will reduce the size and weight of the sieve beds it will also reduces the service life of the sieve bed. Oxygen equipment used for Long Term Oxygen Therapy (LTOT) is optimally deployed for 3-5 years without any service requirements. Any complex service requirement within that time interval simply adds to the overall cost of the equipment, which substantially reverses any cost benefit gained by removing a membrane air dryer or pretreatment layer. Further, allowing sieve bed contamination without prevention or service may lead to providing 82-87% purity oxygen instead of 87-95% pure oxygen to the patient. At this time, portable oxygen concentrator adoption will require smaller, lighter devices that do not require complex field service by a technician or equipment provider, but also minimize size and cost of the equipment. However, if water contamination is the main contributor to operational lifetime, changing out the beds and/or adsorbent can be a relatively simple, easy and fast process for a concentrator designed with simple maintenance by the patient or user in mind. For such a case, it is vital that a user know ahead of time when adsorbent change is required so that replacement adsorbers are available and reduced purity oxygen is not delivered to the patient.
A typical portable oxygen concentrator may contain an oxygen purity sensor that alerts the user or equipment provider when the output oxygen falls below a defined concentration. Such oxygen sensors may be fuel cells, ceramic, or ultrasonic in operating principle, but they ultimately are used for the same purpose, which is to monitor the purity of the product oxygen produced by the oxygen concentrator. These sensors meet the regulatory requirements for alarming if the oxygen purity drops, but these sensors are typically only used for that purpose, thus they do not give the patient or equipment provider any insight into the operating health of the equipment. The low oxygen alarm is also a reactive alarm since the purity of the oxygen has already fallen below the predetermined minimum level. In the event of a low oxygen alarm trigger, in the best case the patient will be receiving reduced purity oxygen until a backup oxygen source is used or an equipment provider or technician can replace or repair the concentrator. In any case, the low oxygen alarm creates a situation that requires immediate action by the user and/or the equipment provider. This type of fast response is inherently more costly and stressful than a planned maintenance activity. It is the object of this invention to provide means for predicting remaining service life allowing users of appropriately designed concentrators to proactively perform maintenance before the purity of their oxygen has already reached a critical level.